that deuterium-labeled aryl iodide 4-d furnishes
a 1:1 mixture of diastereomers (Fig. 3C) is fully
consistent with a radical pathway, whereas the
oxidative-addition/syn-insertion/reductive-elimination
sequence should only produce diastereomer 6-d.

A control experiment established that olefin 4-d
does not undergo cis/trans isomerization under the
reaction conditions. We have also performed this
stereochemical study with the bromo analog of 4-d;
again, a 1:1 mixture of diastereomers is generated,
and no uncyclized products are observed.

An additional mechanistic probe that has
been used to distinguish between concerted
oxidative addition of Ar–X and a pathway involving SET to the haloarene is the relative
reactivity of 1-bromonaphthalene (7) and 4-
chlorobenzonitrile (8) (Fig. 3D) (13). According
to this analysis, if C–X cleavage proceeds via
concerted oxidative addition, then preferential
coupling of 1-bromonaphthalene is expected,
whereas if the reaction occurs via an SET mechanism, then 4-chlorobenzonitrile should react more
rapidly because of its more favorable reduction
potential (–2.03 V for 8; –2.17 V for 7 versus SCE
in DMF) (28).

When copper–carbazolide complex 1 is irradiated in the presence of a 1:1 mixture of
1-bromonaphthalene and 4-chlorobenzonitrile,
Ullmann coupling product 10, derived from
4-chlorobenzonitrile, is predominant (Fig. 3D).
This observation is consistent with a radical-based
SET pathway for C–N bond formation and stands
in sharp contrast with a previous investigation in
which only the bromoarene was reactive, which was
interpreted as supporting a concerted mechanism
for oxidative addition under those conditions (13).

Because copper-catalyzed Ullmann C–N cou-plings are of substantial interest (6–9), we havepursued preliminary studies to ascertain whetherturnover can be achieved in these photoinducedprocesses. We have determined that irradiation ofiodobenzene and lithium carbazolide in an aceto-nitrile solution of 10 mole percent (mol %) ofcopper–carbazolide complex 1 does indeed fur-nish the C–N coupling product in 64% yield,establishing the viability of copper catalysis inthis photochemical reaction manifold (Table 1B,entry 1). In the absence of light, no detectablecoupling is observed (Table 1B, entry 2), and ir-radiation of the coupling partners in the absenceof complex 1 leads to very little N-phenylcarbazole(3%) (Table 1B, entry 3). These copper-catalyzed,photoinduced Ullmann couplings can even be ef-fected at –40°C (Table 1B, entry 4). In the pres-ence of 1.5 mol of copper–carbazolide complex1, a turnover number of ~20 can be achieved(Table 1B, entry 5). CuI also serves as a catalystfor photoinduced Ullmann C–N couplings, likelyvia electron transfer from a luminescent copper–carbazolide complex generated in situ (Table 1B,entries 6 and 7).